Key elements of a fuel cell stack include an anode, a cathode, an electrolyte, and current collectors. In many fuel cell stack designs the electrolyte is sandwiched between the anode and cathode to form the basic cell stack, which may be repeated in series and/or parallel within a fuel cell. This cell stack sandwich is connected to two of the current collectors, which conduct electrical current to and from the electrodes. The current collectors aim to leave as much of the electrode surfaces exposed for gas exchange as possible.
Fuel cell types include among others, solid oxide fuel cells (SOFCs—to be described more fully immediately below), proton conducting ceramic fuel cells, alkaline fuel cells, polymer electrolyte membranes (PEM) fuel cells, molten carbonate fuel cells, solid acid fuel cells, and direct methanol PEM fuel cells.
SOFCs are among the most efficient of all fuel cell types, using ceramic materials for some of the active fuel cell components. A typical anode, for example, is made of electrically conducting nickel/yttria-stabilised zirconia cermet (Ni/YSZ), but the anode and cathode could be formed from any suitable material, such as nickel or lanthanum chromate, as desired and/or necessitated by a particular end use. Various exemplary anodes and/or cathodes can be metal(s), ceramic(s) and/or cermet(s). Some non-limitative examples of metals which may be suitable for the exemplary anode include at least one of nickel, platinum and mixtures thereof. Some non-limitative examples of ceramics which may be suitable for an anode include at least one of CexSmyO2-∂, CexGdyO2-∂, LaxSryCrzO3-∂, and mixtures thereof. Some non-limitative examples of cermets which may be suitable for an anode include at least one of Ni-YSZ, Cu-YSZ, Ni-SDC, Ni-GDC, Cu-SDC, Cu-GDC, and mixtures thereof.
SOFCs are truly solid state since they require no liquid phase to transport charged anions from one electrode-electrolyte interface to the other. SOFCs can reduce production costs by simplifying design since corrosion is not a concern and the electrolyte has no parts or phases that need replacing: solid electrolytes can crack, but they cannot leak as there are no liquid species present. SOFCs are typically operated around 900-1000° C., however, cooler SOFCs are also available.
Some non-limitative examples of metals which may be suitable for a cathode include at least one of silver, platinum and mixtures thereof. A typical cathode may be made of a perovskite, lanthanum manganate (LaMnO3). Some non-limitative examples of ceramics which may be suitable for a cathode include at least one of SmxSryCoO3-∂, BaxLayCoO3-∂, GdxSryCoO3-∂.
A typical solid oxygen conducting electrolyte is made of yttria-stabilised zirconia (YSZ), but the exemplary electrolyte may be formed from any suitable electrolytic material. Various exemplary electrolytes include oxygen anion conducting membrane electrolytes, proton conducting electrolytes, carbonate (CO32−) conducting electrolytes, OH− conducting electrolytes, and mixtures thereof.
Other exemplary electrolytes include cubic fluorite structure electrolytes, doped cubic fluorite electrolytes, proton-exchange polymer electrolytes, proton-exchange ceramic electrolytes, and mixtures thereof. Other exemplary electrolytes can be samarium doped-ceria, gadolinium doped-ceria, LaaSrbGacMgdO3-∂, and mixtures thereof, which may be particularly suited for use in SOFCs.
To generate a suitable voltage, fuel cells in the same stack are often interconnected with a doped lanthanum chromate (e.g., La0.8Ca0.2CrO3) joining the anodes and cathodes of adjacent units. Although there are several stack designs, a common design is the planar (or “flat-plate”) SOFC.
The fuel flow for an exemplary fuel cell may contain a hydrocarbon fuel suitable for generating electricity in a dual chamber fuel cell, for example, methane (CH4), hydrogen (H2), or other hydrocarbon fuels suited to particular electrode compositions used in fuel cells, i.e., ethane, butane, propane, natural gas, methanol, and even gasoline. Methane and hydrogen are shown in the illustration as representative fuels.
At the anode the fuel adsorbs to the anode surface(s), which are usually porous, and diffuses toward the anode-electrolyte interface. At the cathode, oxidizer molecules, such as oxygen (O2) from air, adsorb to the surface(s) of the cathode, which is also usually porous, and diffuse toward the cathode-electrolyte interface.
In a typical oxygen anion electrolyte, as the oxygen molecules diffuse toward the cathode-electrolyte interface, they become exposed to incoming electrons from the cell's external electrical circuit, and capture the electrons to become oxygen anions (O−2). The oxygen anions migrate by toward the positively biased anode-electrolyte interface. When the oxygen anions and the fuel meet at the anode-electrolyte interface, the fuel combines with oxygen anions—an oxidation reaction—to form reaction products, such as water and carbon dioxide. Electrons are left over once the reaction products have formed. Two electrons are left over each time an oxygen anion combines with either a carbon atom or two hydrogen atoms of the fuel. The lost electrons are the source of the electric current that may be harnessed via the cell's external electrical circuit. Water and carbon dioxide diffuse toward the outer surface(s) of the anode and return to the stream of fuel flow.
In many types of conventional fuel cells, performance is decreased when manufacturers increase the thickness of an electrode or an electrolyte in order to use the electrode or the electrolyte as a mechanism for physically supporting the entire cell stack within a fuel cell chamber. A dense but thin electrolyte and porous but thin electrodes are desirable for efficient performance of many types of fuel cells, such as SOFCs. Thinner electrodes and electrolytes, however, would compromise structural integrity.